(1) Fields of the Invention
The present invention relates to a process for manufacturing seamless metal tubes by a cross-roll helical rolling process such as Mannesmann mandrel mill process or by a press piercing process such as Ugine Sejournet process, and more particularly to a process which makes it possible to equalize wall thickness or to correct wall eccentricity.
(2) Description of the Prior Art
A rolling process to reduce outside diameter of hollow shells after piercing billets is known (Japanese Patent Publication Kokai No. 55-103208). The process has a purpose to decrease the number of sizer of billets to be provided to meet the specifications of various different finished products. A 2-roll type rotary mill is used for the process, but any internal tool such as plug or mandrel bar is not used. A phenomenon is found in said process without any internal tool that wall thickness becomes thick at every circumferential position and thickening phenomenon is more remarkable at thinner position as at thicker position when hollow shell has wall eccentricity. Therefore, it is a possibility to use this phenomenon for wall thickness equalization of shells, and actually the inventor of the present invention confirmed the effect. But any substantial effect was not obtained for tubes whose t/D (the ratio of wall thickness to outside diameter) is 5-15%. Because outside diameter reduction for said tubes with small t/D is 20% at the most, and the effect obtained from such reduction is as small as one obtained by improvement of conventional arts, and, therefore an introduction of said rotary mill does not pay for the purpose of wall thickness equalization or wall eccentricity correction.
On the other hand, it is found by this inventor's experiments that a 3-roll type rotary mill can substantially effectuate wall thickness equalization for shells with t/D of 5-15%, that is, a 3-roll type rotary mill is much more effective to wall thickness equalization as a 2-roll type rotary mill. However, the inventor found out a new problem, that is, on the bottom side of the shells rolled by the rotary mill, there is often caused pentagon formation as shown in FIGS. 9 and 10. The smaller the t/D, and the larger the ratio of outside diameter reduetion, the more noticeable the phenomenon is. What is worse, as rolling speed is higher, pentagon formation extends over a larger length. Therefore, it is indispensable to solve this problem in order to apply said wall thickness equalization by the rotary mill for mass production line.